1. Field of the Invention
This invention relates to a device to reduce pulsations on hydrostatic positive displacement units which are either axial or radial piston machines used both as a pump and as a motor, with a reversible direction of rotation. In these displacement units at least one piston is mounted so that it can move longitudinally in a cylinder bore forming a cylinder chamber. The device of the present invention has a buffer element which is in communication with the high pressure side of the displacement unit and which can be brought into communication with the cylinder chamber of the displacement unit.
2. Background Information
Hydrostatic positive displacement units generally have a plurality of cylinder chambers and deliver a non-constant, pulsating volume flow. One of the causes of these pulsations in the flow of the positive displacement unit is the result of the kinematic conditions. On a pump, the hydraulic fluid is transported by several pistons movable longitudinally in cylinders and work according to the positive displacement principle from the low pressure inlet side to the high pressure outlet side. As a result of the superimposition of the individual volume flows to form the total volume flow of the positive displacement unit, there is a pulsation in the flow being transported. This type of pulsation is designated a kinematic pulsation.
An additional cause of the pulsations is the kinetic pulsation which originates from the compressibility of the medium being transported, and which occurs primarily when there are large pressure differentials between the inlet side and the outlet side. This type of pulsation is caused by pressure equalization currents which occur during the reversal actions of the cylinder chambers from the inlet side to the outlet side. If, for example, a cylinder chamber of a rotating cylinder drum is moved from the low pressure inlet side to the high pressure outlet side at the corresponding dead center position of the movement of the piston, the cylinder chamber traverses an area in which the cylinder chamber is briefly not in communication with either the low pressure side or the high pressure side. When communication is subsequently established between the cylinder chamber and the high-pressure side, volume flows occur as a result of the pressure differential between the cylinder chamber and the high-pressure side. As the cylinder chambers move further, the cylinder chamber also traverses an area in which the cylinder chamber is not connected to the high-pressure side or the low-pressure side. Large pressure differentials also occur when the cylinder chamber is in communication with the low-pressure side. Consequently, pulsations originate which result in vibrations and noises in the positive displacement unit.
To reduce pulsation, the prior art discloses the use of a buffer element which effects an equalization between the pressure in the cylinder chambers and the pressure on the high-pressure side.
A hydrostatic axial piston machine with such a buffer reversal is described in DE 42 29 544. On the machine, there is a buffer element in the form of an oil-filled pre-compression space which is placed in communication with the cylinder chamber after the cylinder chamber has passed the dead center position by means of a connecting channel and an opening in the control plate. Hydraulic fluid is thereby extracted from the pre-compression space, as a result of which the pressure in the cylinder increases. The pre-compression space is filled via a line which is in communication with the high-pressure side of the machine.
The pre-compression space is supplied with fluid via a constant connection between the pre-compression space and the outlet side of the machine. If a cylinder space moves from the inlet side to the outlet side, and if low pressure is applied to the inlet side and high pressure to the outlet side, hydraulic fluid is extracted from the pre-compression space as soon as the cylinder chamber has exposed the opening in the control plate. As a result of this measure, the pressure in the cylinder chambers is equalized to the pressure of the outlet side, whereupon lower volume flows are formed to equalize the small remaining pressure difference when the cylinder chambers are connected to the outlet side. With this measure, however, a specially designed cylinder nodule is required to connect the cylinder chambers with the pre-compression space, to make it possible for the hydraulic fluid to flow rapidly from the pre-compression volume into the cylinder chamber.
The prior art also includes the recharging of the pre-compression space during the period in which the cylinder space is in communication with the high-pressure side. To fill the pre-compression space, therefore, only a temporary communication with the high-pressure side is established. For this purpose, a specially shaped cylinder nodule is required. While the cylinder chamber is in communication with the pre-compression volume, this cylinder nodule first briefly establishes communication between the cylinder chamber and the pre-compression space by means of a connecting channel. During this period, the pressure in the cylinder is increased. As the cylinder chamber moves farther toward the outlet side, the communication between the cylinder chamber and the buffer element is interrupted. In a further phase, an increasingly large cross section is formed which makes it possible to fill the pre-compression space as soon as the cylinder is in communication with the high-pressure side and the connecting channel.
The volume flow which is required to fill the pre-compression space is defined by a throttle which is located in a channel which connects the pre-compression space with the cylinder chamber. The selection of the throttle has a significant influence on the pulsation action of the positive displacement unit. With a severe throttling, the volume current which flows to the pre-compression space will be small, and thus the pre-compression space is not filled at the pressure applied on the high-pressure side. As a result, the volume flow into the cylinder chambers will also be small, which means that the pressure equalization of the cylinder chambers will be insufficient. If the throttling of the volume flow is small, the pre-compression space can no longer be considered a pilot element, but forms a part of the high-pressure side, as a result of which the pulsation-reducing action is lost. The selection of the throttle used to fill the pre-compression volume is therefore also determined by the volume flow flowing from the pre-compression space to the cylinder chamber, and by the pulsation behavior of the positive displacement unit.
In these measures with a buffer reversal, the cylinder chamber is placed in communication with the pre-compression space only briefly. Only a short period of time is therefore available for the required pressure equalization. The time during which the cylinder chamber is in communication with the space via the connecting channel is controlled by the geometry of the connecting line and of the cylinder nodule. The optimum opening time must thereby be considered the time in which a pressure equalization can take place between the cylinder chamber and the pre-compression space. This opening time, however, is a function of the operating parameters, such as the speed of rotation, the operating pressure and the displacement position. The opening time with these measures is defined, however, by the geometry of the components, which means that an effective reduction of pulsations is not achieved under all operating conditions.